This is not "Which boss is strongest" This is based on the level that you are at the time you faced them, your personal difficulty with them, crap like that. Personally I never found Faust or Frhama to be that difficult, just emperor doll believe it or not. (Low level when I faced him.)

The final Boss was my Hardest fight.I ran away a lot and never really trained.By the time I saw Melbu's ugly face I had Half of Dart's Volcano addition.I was owned, REPEATEDLY.Took me weeks to figure out that Rose Storm was specifically crafted for morons who don't train.